A variety of analytical techniques are used to characterize interactions between molecules, particularly in the context of assays directed to the detection of biomolecular interactions by use of a biosensor. For example, antibody-antigen interactions are of fundamental importance in many fields, including biology, immunology and pharmacology. In this context, many biosensor-based analytical techniques involve binding of a "ligand" (such as an antibody) to a solid support on a sensing surface (i.e., a sensitized surface on a sensor chip), followed by contacting the surface-bound ligand with an "analyte" (such as an antigen). The surface-bound ligand is generally contacted with the analyte by "flowing" directly adjacent thereto an analyte solution, where the analyte solution flows within a flow channel or conduit structure of the biosensor.
In this regard, there are a number of different classes of biosensor instrumentation designed to contact analyte and reagent solutions with one or more sensing surfaces. One representative class includes the affinity-based biosensors manufactured and sold by Biacore AB (Uppsala, Sweden)(hereinafter the "Biacore instrument"). The Biacore instrument utilizes surface plasmon resonance (SPR) technology and, in its simplest form, includes a light source such as a light emitting diode, a sensor chip covered with a thin gold film, a flow-cell conduit system for flowing sample and reagent solutions adjacent to a sensing surface on the sensor chip, and a photo detector. Incoming light from the diode is reflected in the gold film and detected by the photo detector. At a certain angle of incidence ("the SPR angle"), a surface plasmon wave is set up in the gold layer, which is detected as an intensity loss or "dip" in the reflected light. The change of light intensity is plotted as a "sensorgram." The theoretical basis behind such instrumentation has been fully described in the literature (see, e.g., Jonsson, U. et al., Biotechniques 11:620-627, 1991). One of the important features of this class of biosensors is that they are capable of detecting biomolecular interactions in real-time, without the need for radioactive labels or fluorescent tags. As a result, affinity-based biosensors are, in general, becoming much more popular in many biochemical research settings.
The SPR-based Biacore instrument employs optical-based sensors, which may be generally classified as a mass detection method. Other types of mass detection techniques include, but are not limited to, piezoelectric, thermo-optical and surface acoustic wave (SAW) methods. In addition, to mass detection methods, biosensor instruments may also employ electrochemical methods, such as potentiometric, conductometric, amperometric and capacitance methods. However, regardless of the detection method utilized in a specific instrument, sample must be brought into contact with the sensing surface and, after the detection is complete, must be removed from the sensing surface to permit analysis of additional sample, and/or to permit cleaning or regeneration of the sensing surface.
Methods for contacting a sample to a biosensor surface varies widely. For example, the Biacore instrument delivers sample to the sensor chip by means of an integrated microfluidic cartridge. A typical microfluidic cartridge consists of a series of precision-cast channels in a hard silicon polymer plate which forms one or more flow paths for buffer and sample delivery. A set of pneumatically actuated diaphragm valves directs fluid flow through the various channels to the sensing surfaces of the biosensor. In this manner, single or multichannel analysis is permitted. Other sample delivery techniques, such as "hydrodynamic injection" and "continuous flow injection," typically include tubing along with appropriate pumps, or aspiration of the sample onto the sensor surface.
Regardless of the sample delivery technique employed, the injection performance in terms of low dispersion is correlated to minimizing the dead volume. In commercially available systems, like the Biacore instrument, the "dead volume" is defined by the distance (which equates to volume) between the detector and the valve; the further away the valve is from the sensing surface the greater the dead volume (see, e.g., Ruzicka J. and Hansen E., Flow Injection Analysis, John Wiley & Sons, New York, 1988).
Although a number of techniques have been employed to deliver sample to the surface of a biosensor, as well as to remove sample from the surface, there is still a need in the art for improvements to such delivery systems. For example, an improved sample deliver system should be capable of contacting sample and/or reagent solutions to one or more sensing surfaces of the biosensor so as to optimally conserve sample and/or reagent solutions. Moreover, reducing the dead volume or the time delay associated with contacting a plurality of solutions, such as buffer, sample, and regeneration solutions, with the one or more sensing surfaces of the biosensor would be desirable.
Accordingly, there is a need in the art for an improved biosensor flow conduit systems, as well as to methods and apparatus related thereto. The present invention fulfills these needs, and provides further related advantages.